The present invention relates to valve construction. More particularly, the invention relates to an improved bypass/relief valve integral with a potable liquids pump.
Vane type pumps are often used by the beverage, food and health industries for quietly transferring potable liquids at moderately high pressures and volumetric flow rates. For example, vane pumps are used to spray water into a carbon dioxide atmosphere tank at 200 to 250 psi as a step in the production of carbonated beverages.
One particular design of vane pump provides a shunt flow channel between the suction and discharge conduits of the pump. Flow through this shunt is differentially controlled by coaxial valve elements. An axial bore along the relief valve plug element is flow directionally rectified by a spring biased poppet element at the discharge port end of the shunt. This, in effect, is a check valve with a spring loaded cracking bias by which fluid flow bypasses the pump rotor from the suction port directly into the discharge port when suction side pressure exceeds the pump discharge port pressure.
The discharge port relief valve plug is held against a closure seat by an adjustably loaded compression spring that opposes the pump discharge pressure. Compression preload on the relief valve spring is adjusted by an axial displacement of the spring seat. The spring is seated on a screw threaded plug which is advanced or retarded by manual rotation. When the discharge pressure forces on the plug seat cross-sectional area exceed the preload force of the seating spring, the plug moves away from the valve seal face thereby permitting pump discharge fluid flow to recirculate directly back into the pump suction conduit.
Pumps manufactured for sanitary applications such as in fast food restaurants, hospitals or for food processing frequently are made with brass bodies and use brass plugs in the relief valve conduit. While this design has generally performed well for many years, there is a recent demand for greater pump reliability, especially in soft drink fountain applications. It has been determined that the relief valve in this pump is a statistically substantial source of failure. The metal-to-metal valve element combination with cooperating soft metals tends to leak a minute amount but such leakage has little or no apparent effect on pump operation. Once leakage flow begins, erosion exacerbates the defect and the initial small leak develops into a catastrophic failure effectively rendering the pump unusable.
A different cause of the same result arises from inaccurate or defective machine tools that turn, bore and cut imprecise or irregular shapes and surfaces. Relative to the present interest, if a relief valve seat or plug face is out-of-round the pair cannot cooperate to form a fluid-tight seal and small leaks quickly grow into large leaks.
It is an objective of the present invention, therefore, to provide a relief valve design and construction for potable fluid vane pumps having greater accuracy in the shape of the valve sealing surfaces.
Another object of the invention is to provide a relief valve design and construction in which one of the valve sealing elements is significantly softer than the other seal element.
A further object of the invention is to disclose a valve plug material that, relative to the valve seat, is softer and tougher.